A so-called Co-added type ferromagnetic iron oxide is used as ferromagnetic particles suitable for a magnetic recording medium such as a video tape and the like. The Co-added type iron oxide can be made by preparing acicular iron oxide particles (although the term "acicular iron oxide particles" as used herein mainly means ".gamma.-Fe.sub.2 O.sub.3 ", partly means also "Fe.sub.3 O.sub.4 "), for example, obtained by adsorbing amorphous cobalt compounds such as cobalt hydroxide (Co(OH).sub.2) onto acicular iron oxyhydroxide (.alpha.-FeOOH, hereinafter called "goethite"), washing with water, filtering, drying and calcining The calcining step generally is conducted at comparatively high temperature such as 300.degree. to 500.degree. C., and includes a dehydration step, a reduction step which results in the formation of Fe.sub.3 O.sub.4, and, if desired, an oxidation step which results in the formation of .gamma.-Fe.sub.2 O.sub.3. During calcining, the cobalt ions diffuse into the acicular iron oxide particles that are being formed. Hereinafter, the above method is referred to as the "Co-doped method" as described in U.S. Pat. Nos. 4,297,395 and 3,903,004.
The process of the Co-doped method is comparatively simple and has comparatively low manufacturing cost, but it has a number of defects, such as dependence on temperatures is high, for example, Hc (coercive force) changes about 100 Oe with a 10.degree. C. difference, demagnetization occurs to a great extent when pressure is applied or when heating takes place, and a print through phenomenon that signals recorded on a part of the magnetic layer are transferred on the other part easily occurs in a magnetic recording medium prepared by using the above-described magnetic particles.
In order to overcome these defects, it has recently been proposed and disclosed, for example, in Japanese Patent Publication Nos. 49475/74 and 29157/75, and Japanese Patent Application (OPI) Nos. 74399/74 and 37667/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and U.S. Pat. No. 4,122,216 that .gamma.-Fe.sub.2 O.sub.3 particles are coated with Co, or are modified by Co to prepare a ferromagnetic iron oxide instead of diffusing Co into .gamma.-Fe.sub.2 O.sub.3 particles.
These magnetic particles are prepared by first forming an acicular magnetic iron oxide (mainly .gamma.-Fe.sub.2 O.sub.3), water washing, filtering, drying and calcining (dehydration, reduction and, if desired, oxidation) an aqueous suspension of, for example, acicular goethite particles, and then adding water-soluble cobalt salts, ferrous salts and alkali and the like into the aqueous suspension thereof, which is called a wet reaction. Hereinafter, this method is referred to as the "Co-modified method".
The Co-modified ferromagnetic iron oxide particles prepared by the above Co-modified method have such favorable characteristics that these iron oxide particles have high coercive force, dependence on temperatures is low (i.e., there is about a 20 Oe change with a 10.degree. C. difference), demagnetization caused by adding pressure or by heating is low, and there is hardly any print through phenomenon. However, the manufacturing process is complicated, the manufacturing time is long and the manufacturing cost becomes also high, because as is clear from the above-described process, acicular magnetic iron oxide (mainly .gamma.-Fe.sub.2 O.sub.3) is first formed, and then is Co-modified by the wet reaction.